Disable thread_local used in Progress on OS X if Xcode does not support it,...

Disable thread_local used in Progress on OS X if Xcode does not support it, since Xcode 7 does not. Progress::Context in turn becomes a static global variable.

Progress/ProgressNode.hh

@@ -5,7 +5,14 @@
 
 #ifdef PROGRESS_ON
 namespace Progress {
-
+    
+#if defined(__apple_build_version__)
+// work around the lack of thread_local support in XCode 7
+#if !__has_feature(cxx_thread_local)
+#define thread_local // disable thread_local
+#endif//!__has_feature(cxx_thread_local)
+#endif//defined(__apple_build_version__)
+    
 //! Progress tick counter type
 typedef unsigned long long TickNumber;